SE-B-8307139-9 (GKN-Stenman) describes a lock of this kind in which one of the intermediate pins has a larger diameter than a corresponding upper pin in the first row of pin-channels and a corresponding pin-channel in the plug has a widened part capable of accommodating said intermediate pin of larger diameter. In this case, the intermediate pin can be accommodated in the widened part of the plug pin-channel, but is unable to enter plug pin-channels of smaller diameter, and hence the service key is latched. This solution is simple from the aspect of manufacture and affords several advantages. However, if an attempt is made to remove a service key with the lock in its normal or standard mode, there is a risk that the intermediate pin will wedge between a widened and a narrower channel part, therewith making it impossible to return the plug to the service mode of the lock. This means that the service key cannot be removed under any circumstance and that the lock must be broken into, in order to open the lock.
NO-A-793880 (Elkem-Spigerverket) describes another lock arrangement in which the number of pin-channels in the standard mode of the lock differs from the number of pin channels in the service mode thereof. In this case, withdrawal of the service key when the lock is in its standard mode is prevented, because of the inability of a corresponding lower pin to move upwards in the service mode of the lock. It is comparatively simple, however, to file down the part of the key concerned, so as to negate the intended latching effect.
Lock constructions are also known to the art in which the plug is intended to move axially in relation to the plug housing, when the plug is turned to lock or unlock the lock mechanism. These lock constructions, however, do not have the aforesaid special function which enables the lock to be opened with the aid of a service key. Examples of known lock constructions which incorporate an axially movable plug are found in DE-A-26 14 645. (Sanpo Lock), U.S. Pat. No. 4,099,395 (Garza) and U.S. Pat. No. 3,264,852 (Gysin).